Notre Dame Cathedral would have burned to the ground in a “chain-reaction collapse” had firefighters not moved as rapidly as they did, according to an official.

Firefighters acted aggressively to protect wooden supports in the twin medieval bell towers from the flames, averting a bigger catastrophe, said Jose Vaz de Matos, a fire expert with France’s Culture Ministry.

“If the fire reached this wooden structure, the bell tower would have been lost,” Mr de Matos said at a news conference. “From the moment we lose the war of the bell towers, we lose the cathedral, because it’s a chain-reaction collapse.”

The fire destroyed most of the lead roof of the 950-year-old architectural treasure and caused its spire, which was added in the 19th century, to collapse.

An initial fire alert was sounded at 6.20pm, as a Mass was under way in the cathedral, but no fire was found.

The second alert was sounded at 6.43pm, and the blaze was discovered on the roof. No one was killed in the fire, after firefighters and church officials speedily evacuated those inside.

Firefighters acted bravely and as fast as they could to save the cathedral, said senior fire official Philippe Demay, denying that there was any delay in their response.

Despite extensive damage, many of the cathedral’s treasures were saved, including Notre Dame’s famous rose windows, although they are not out of danger.

Paris Firefighters’ spokesman Lieutenant-Colonel Gabriel Plus said that even though they are “in good condition … there is a risk for the gables that are no longer supported by the frame”.

Firefighters removed statues inside the gables, or support walls, above the rose windows to protect them, and took care not to spray water too hard on the delicate stained glass, Mr Plus said.

Notre Dame’s towers could have fallen without swift action taken by firefighters, a Paris fire official said (AP Photo/Francois Mori)

Scaffolding erected for a renovation of the spire and roof that was already under way must be properly removed because of its weight and because it is now “crucially deformed”, he added.

The cathedral is still being monitored closely by firefighters and experts to determine how much damage the structure suffered and what needs to be dismantled to avoid collapse.

“The experts are scrutinising the whole of the cathedral, part by part, to identify what is weakened, what will need to be dismantled or consolidated,” Mr Plus said.

Nearly a billion euro has been pledged for the restoration, while a vow by French President Emmanuel Macron to finish it in five years has been challenged as being wildly off track.

He said the renovations would be completed in time for the 2024 Olympics in Paris.

“We will rebuild the cathedral to be even more beautiful, and I want it to be finished within five years,” Mr Macron said.

Experts have said that Mr Macron’s ambitious goal appears insufficient for such a massive operation. Even Prime Minister Edouard Philippe, while supporting the government timeline, acknowledged that it would be difficult.